Nuclear magnetic resonance imaging method and apparatus therefor

ABSTRACT

A plurality of signal acquisition steps  103  to  106  are executed continuously in succession to an inversion longitudinal magnetization generation step  101  for generating inversion magnetization by applying inversion RF pulses to an object. Similar process steps are iterated in another inversion longitudinal magnetization step in a slice non-selecting mode, and the difference is determined between image data acquired by the first signal acquisition steps  103  to  106  and image data acquired by the second signal acquisition steps  107  to  110  to acquire a perfusion image. In this instance, the mode of applying the gradient magnetic field is made different in each signal acquisition step, and images of a plurality of slices are acquired. The process steps described above are iterated while the correspondence relation between an inversion time TI and a selected slice is changed. An image having different inversion times can thus be acquired within a short time for a plurality of slices. Consequently, an image reflecting the time change of perfusion and having a high diagnostic value can be acquired over a broad range.

TECHNICAL FIELD

This invention relates to a nuclear magnetic resonance imaging (MRI)method for measuring nuclear magnetic resonance (hereinafter referred toas “NMR”) signals from hydrogen, phosphorus, and so forth, in an object,and imaging a density distribution of nuclei and a relaxation time. Moreparticularly, the present invention relates to a method of, and anapparatus for, imaging perfusion of distal blood vessels, etc, with highresolution.

BACKGROUND ART

FMRI (Functional MRI) for extracting local activation of the brain froma slight local signal change of time-series MR images has been put intopractical application in recent years. A spin tagging method thatapplies IR (Inversion Recovery) pulses and then executes imaging by EPI(Echo Planar Imaging) sequences has been studied as one of the FMRI, andmethods of applying various IR pulses have been examined, too. Suchprior art technologies include SG. Kim “Qualification of relativecerebral flow change by flow-sensitive alternating inversion recovery(FAIR) technique: application to functional mapping”, Magnetic Resonancein Medicine, 34, pp. 293-301(1995), and SG. Kim et al. “Fast interleavedecho-planar imaging with navigator: high-resolution anatomic andfunctional images at 4 Tesla”, Magnetic Resonance in Medicine, 35, pp.895-902(1996).

FIG. 9 shows an example of the sequences by the spin tagging method.Initially, an inversion radio frequency pulse (the radio frequency pulsewill be hereinafter referred to as the “RF pulse”) 901 is appliedsimultaneously with a slice selecting gradient magnetic field 902. Theinversion RF pulse, that is known as an adiabatic inversion RF pulse, isemployed as the inversion RF pulse 901 because an accurate rectangularexcitation shape can be obtained. A slice becomes thinner when theintensity of the slice selecting gradient magnetic field 902 is higherand becomes thicker when the intensity is lower. When the intensity iszero, that is, when no gradient magnetic field is applied, no slice isselected. A region having a desired thickness can thus be excitedselectively (tagging).

After the passage of a predetermined time TI 905 from the end of theapplication of this inversion RF pulse, an EPI sequence 904 shown inFIG. 2 is executed. In this EPI sequence 904, after the RF pulse 201 isapplied simultaneously with the slice selecting gradient magnetic field202, a phase encode gradient magnetic field 203 is applied in the pulseform so that the application amount is different each time. At the sametime, a gradient magnetic field 205 in a readout direction is appliedrepeatedly and inversely so as to measure echo signals 207. Referring toFIG. 9, a time TI 905 from the end of the application of the inversionRF pulse 901 to the irradiation timing of the RF pulse of an imagingsequence 904 is referred to as a “time of inversion”.

In such a spin tagging method, while magnetization of the regionselected by the inversion RF pulse is inverted (tagged), spins ofbloodstream, etc, migrate from other regions. In consequence, thesignals containing the tagged spins and the non-tagged spins in mixturecan be acquired in the imaging sequence. Images of perfusion can beacquired by utilizing these signals.

FIG. 10 shows an example of the imaging sequence by applying the spintagging method to perfusion imaging. This imaging sequence executesonce, or iterates continuously, the sequence shown in FIG. 9 in apredetermined repetition time Tr 906. In this instance, the firstinversion RF pulse (IR pulse 101) is for slice selection and the secondinversion RF pulse (IR pulse 102) is for slice non-selection. Thedifference of the image data obtained by these two imaging sequences iscalculated, and the local perfusion of the brain in the slice selectingregion (slice 1) can be imaged. Assuming that the repetition time Tr 906is 2 seconds and the inversion time TI 905 is 1 second in this case, anabout 3-second time is necessary because an ordinary single-shot EPIsequence needs about 100 msec time. One image can be acquired for oneslice in the predetermined inversion time.

In the slice selecting RF pulse 903 and imaging (904) of the slice 1 inFIG. 10, the echo signals obtained by imaging are processed by knownimage processing (such as two-dimensional Fourier transform), giving theimage of the slice 1. This image is generally referred to as the “IR(Inversion Recovery) image”. It is known that the IR image has varioustexture contrasts depending on the size of TI. When the artery flowsfrom below to up in the object in this image (for example, the cerebralartery branching from the main artery when the transaxial image of thehead is imaged), signals resulting from the bloodstream (perfusion) areimparted to the IR image. In imaging of the slice non-selecting RF pulseand the slice 1 shown in FIG. 10, too, the echo signals obtained byimaging are processed by known image processing (such as two-dimensionalFourier transform), giving the image of the slice 1. This image is alsoa kind of the IR (Inversion Recovery) images. When the artery flows frombelow to up in the object, however, the signals resulting from thebloodstream (perfusion) of the blood inverted by the IR pulses areimparted to the IR image. The difference of the data between the firstimage and this image is calculated, and the signals resulting from theobject itself can be removed. Inconsequence, only the signals thatdepend on the bloodstream (perfusion) are imaged.

In order to obtain this IR image, it has been necessary in the past toapply the IR pulses whenever the slice is changed.

In the spin tagging method described above, the inversion time TI fromthe end of the application of the inversion RF pulse to the applicationof the RF pulse of the imaging sequence reflects the regions at whichthe blood arrives during that period of time. Therefore, when setting ofthe inversion time TI is changed, the image changes, as well. Inperfusion imaging by the existing spin tagging method, imaging isconducted by setting appropriately the inversion time TI (for example,to 0.75 seconds) in accordance with the velocity of the bloodstream tobe observed. In order to observe the region of a diseased part having acertain expansion such as the cerebral apoplexy, it is preferred thatthe TI time can be set in various ways to acquire the images.

It has been required to acquire not only one slice but also multipleslices, or three-dimensional images. When the images each having avarious TI time are acquired for the multiple slices, for example, theimaging time of (Tr+TI+EPI imaging time)×(number of TI settingtimes)×(number of slices) is necessary as the time required for a seriesof imaging, and the imaging time gets elongated.

DISCLOSURE OF INVENTION

In perfusion imaging by utilizing the spin tagging method, therefore, itis an object of the present invention to provide an MRI method, and anapparatus for the method, that can image the moving condition of abloodstream, the flow velocity of which changes as it flows towardsdistal ends, over a broad range, and can acquire multi-slices orthree-dimensional images within a short imaging time.

It is another object of the present invention to provide an MRI method,and an apparatus for the method, that can acquire perfusion images withhigh spatial resolution.

An MRI method according to the present invention for accomplishing theobjects described above comprises an inversion longitudinalmagnetization generation step for generating inversion longitudinalmagnetization by applying radio frequency pulses to an object containingmagnetization to be detected, and an imaging step for executingcontinuously a plurality of signal acquisition steps in succession tothe inversion longitudinal magnetization generation step.

Here, the signal acquisition step may comprise a sequence capable ofacquiring a plurality of signals by one-time excitation ofmagnetization, and includes a step for irradiating at least one RF pulseto an object containing magnetization to be detected and generatingtransverse magnetization in a selected slice, a step for generating aplurality of echo signals by applying gradient magnetic fields to theobject having the transverse magnetization applied thereto, and a stepfor detecting a plurality of echo signals in a time series.

The step for generating a plurality of echo signals and the step fordetecting of a plurality of these echo signals in a time series mayinclude, for example, a step for applying readout gradient magneticpulses, that inverse continuously, to the object to which the transversemagnetization is applied, a step for applying phase encode gradientmagnetic fields in synchronism with the readout gradient magnetic fieldpulses, and a step for detecting a plurality of echo signals generatedin each cycle of the inverting readout gradient magnetic fields in atime series (EPI: Echo Planar Imaging). The step for generating thetransverse magnetization may be the one that irradiates a plurality ofRF pulses, irradiates further second RF pulses for inverting the phaseof the transverse magnetization, and acquires the same number of echosignals as the number of a plurality of RF pulses (burst sequence). Thesignal acquisition step, in particular, is most appropriately the onethat irradiates a plurality of RF pulses while a gradient magnetic fieldin a readout direction is being applied, then irradiates 180° RF pulse(for inverting the phase of the transverse magnetization) while a sliceselecting gradient magnetic field is being applied, generates seriallythe same number of sets of echo signals as the number of RF pulses whilethe gradient magnetic field in the readout direction is being inverted,applies in this instance a different phase encode gradient magneticpulse whenever the gradient magnetic field inverts, and detects aplurality of sets of echo signals in a time series (hybrid burstsequence).

In the first embodiment of the present invention, mutually differentslice positions are selected and signals are acquired by a plurality ofsignal acquisition steps that are executed in succession to theinversion longitudinal magnetization generation step. In an embodimentof the present invention that is particularly preferred, a plurality ofsets, each of which comprises the inversion longitudinal magnetizationgeneration step and an imaging step including a plurality of signalacquisition steps, are iterated, and in the iteration of such sets, theslice position for selecting a plurality of signal acquisition steps isserially changed.

In this way, a plurality of images each having a different inversiontime TI from the end of the inversion longitudinal magnetizationgeneration step to the irradiation of the RF pulse in each signalacquisition step can be acquired within an extremely short period oftime for a plurality of slices. Consequently, the change of thebloodstream can be observed over a broad range.

In the second embodiment of the present invention, the same sliceposition is selected in a plurality of signal acquisition steps, and aplurality of images each having a different inversion time TI from theend of the inversion longitudinal magnetization generation step to theirradiation of the radio frequency pulses in each signal acquisitionstep are acquired. In this case, the change of the bloodstream can beobserved with high time resolution.

In this second embodiment, too, the signal acquisition step may be themethod that can acquire a plurality of signals by one-time excitation ofmagnetization. For example, EPI, a burst sequence or a hybrid sequence,can be employed. Furthermore, it is possible to employ a method thatacquires the signals to which a different phase encode gradient magneticfield is applied in each signal acquisition step (split EPI ormulti-shot EPI, burst, etc). In this case, one image can bere-constructed by a plurality of signal acquisition steps. Therefore, animage having high spatial resolution can be acquired.

The third embodiment of the present invention employs three-dimensionalimaging for a plurality of signal acquisition steps. The signalacquisition steps in this case include a step of irradiating at leastone RF pulse to an object containing magnetization to be detected andgenerating transverse magnetization, a step of applying a slice encodegradient magnetic field to the object to which transverse magnetizationis applied, a step of applying mutually different phase encode gradientmagnetic fields to the object to which transverse magnetization isapplied, and generating a plurality of echo signals, and a step ofdetecting a plurality of echo signals in a time series. Means foracquiring a plurality of signals applies mutually different slice encodegradient magnetic fields and acquires the signals.

This embodiment can acquire an image having higher spatial resolutionthan multi-slice imaging in the first and second embodiments thatexecutes the MRI method for a plurality of slices. However, theinversion time TI of the image obtained in this case is a mean value ofthe inversion time TIs of a plurality of signal acquisition steps.

The MRI method and apparatus according to the present invention canacquire within a short time the images reflecting the time change ofperfusion over a relatively broad range and having high diagnostic valueby executing continuously a plurality of signal acquisition steps insuccession to inversion magnetization generation in perfusion imaging byspin tagging.

Because the MRI method and apparatus according to the present inventioncan continuously image the images each having a different inversiontime, they can image the perfusion both spatially and time-wise.Particularly when the images each having a different inversion time isacquired, the images having different inversion time can be imagedcontinuously for a plurality of slices within an extremely short time byserially changing the correspondence relation between the slice to beselected and the inversion time.

The number of slices, the number of kinds of inversion time TI andspatial resolution can be increased within the same signal acquisitiontime by using an ultra-high speed burst sequence that comprises thecombination of a plurality of RF excitation pulses and inversiongradient magnetic fields as the signal acquisition steps. Therefore, theperfusion in a broader region can be observed with higher time andspatial resolution. Consequently, the present invention can provide theimages having higher diagnostic value for blood vessel diseases havingthe lesions in a broader range, such as the cerebral apoplexy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A shows an embodiment of the present invention.

FIG. 1B shows image data obtained in an imaging step of FIG. 1A.

FIG. 2 is a timing chart of an EPI sequence employed in an MRI method ofthe present invention.

FIG. 3A shows an embodiment of the present invention.

FIG. 3B shows an image obtained by an imaging step of FIG. 3A.

FIG. 4 is a block diagram of an MRI apparatus to which the presentinvention is applied.

FIG. 5 shows another embodiment of the present invention.

FIG. 6 shows still another embodiment of the present invention.

FIG. 7 shows still another embodiment of the present invention.

FIG. 8 is a timing chart of a hybrid burst sequence employed in the MRImethod of the present invention.

FIG. 9 shows an example of an imaging sequence according to the priorart.

FIG. 10 shows another example of the imaging sequence according to theprior art.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be explained withreference to the drawings.

FIG. 4 shows the construction of a typical MRI apparatus to which an MRImethod according to the present invention is applied. This MRI apparatusincludes a magnet 402 that generates a uniform static magnetic field ina measurement space in which an object 401 is placed, a gradientmagnetic field coil 403 for generating a gradient magnetic field in thisspace, an RF coil 404 for generating an RF magnetic field in the spaceand an RF probe 405 for detecting MR signals generated by the object401. Abed 412 is provided in order for the object 412 to lie down on.

The gradient magnetic field 403 comprises gradient magnetic field coilsin X, Y and Z three directions, each generating a gradient magneticfield in accordance with a signal from a gradient magnetic field powersource 409. A gradient magnetic field in a predetermined direction isapplied to a static magnetic field in superposition with the latter sothat spins can be selectively excited in a desired region (a slice or aslab which is a thicker region than the slice) of the object 401.

The RF coil 404 generates an RF magnetic field in accordance with thesignals from an RF transmission unit 410. This RF transmission unit 410overlaps a predetermined envelope signal with a carrier wave having apredetermined frequency. In this way, an adiabatic inversion RF pulseused in the present invention for generating inversion longitudinalmagnetization and an RF pulse (excitation pulse) for generatingtransverse magnetization in an image process can be generated.

MR signals induced in the RF envelope 405 are detected by a signaldetection unit 406 and are processed by a signal processing unit 407.They are further converted to image signals by calculation. The image isdisplayed on a display unit 408. Each of the gradient magnetic filedpower source 409, the RF transmission unit 410 and the signal detectionunit 406 is controlled by a control unit 411. The time charts of thesegradient magnetic field, RF magnetic field and signal detection that arecontrolled by this control unit 411 are generally referred to as a“pulse sequence”. In the present invention, a pulse sequence comprisingthe combination of an inversion longitudinal magnetization generationstep and an imaging step comprising a plurality of signal acquisitionsteps is executed as will be explained next.

FIG. 1A shows an embodiment of the MRI method according to the presentinvention. Each step executed in this MRI method is arranged along thetime axis (abscissa). The ordinate represents a slice position of theobject that is excited in each step. This MRI method executes, in timeintervals Tr, an inversion longitudinal magnetization generation stepfor selectively inverting longitudinal magnetization of a predeterminedregion of the object, that is, a slice selecting IR pulse applicationstep 101, a first sequence comprising four signal acquisition steps 103to 106 that are executed continuously in succession to the inversionlongitudinal magnetization generation step 101, an inversionlongitudinal magnetization step for inverting non-selectivelylongitudinal magnetization of all the regions of the object, that is, aslice non-selecting IR pulse application step 102, and a second sequencecomprising four signal acquisition steps 107 to 110 that are executedcontinuously in succession to the inversion longitudinal magnetizationgeneration step 102. When a read step is a single-shot EPI, the set ofthe first and second sequences may be one, but the process is iteratedthe number of times corresponding to the number of shots in multi-shotEPI.

The first inversion longitudinal magnetization step 101 comprises aninversion RF pulse 901 shown in FIG. 9 and a slice selecting gradientmagnetic field pulse 902 that is applied simultaneously with the former.An adiabatic inversion RF pulse capable of providing an accuraterectangular excitation shape is used as the inversion RF pulse 901. Thegradient magnetic field pulse 902 that is applied simultaneously withthe adiabatic RF pulse has an intensity such that the region includingall the slices selected by each signal acquisition step to be executedin succession can be selected. The imaging section is transaxial and theslice thickness is 10 mm, for example.

After the passage of a predetermined time TI1 from the end of theapplication of this inversion RF pulse, the first signal acquisitionstep 103 is executed. Subsequently, the second to fourth signalacquisition steps 104 to 106 are executed. In this embodiment, thesignal acquisition steps 103 to 110 comprise a sequence for acquiring aplurality of signals necessary for reconstructing one image by one-timeexcitation, and execute the single-shot EPI sequence shown in FIG. 2,for example.

In other words, the RF pulse 201 is irradiated to the object containingmagnetization to be detected and at the same time, the gradient magneticfield pulse 202 for selecting the slice is applied. In consequence, theslice to be imaged is selected. Next, a pulse 203 for providing offsetof phase encoding and a pulse 205 for providing offset of a readgradient magnetic field are applied. A read gradient magnetic fieldpulse 206 that inverts continuously is then applied. The gradientmagnetic field pulse 206 is a trapezoidal pulse, for example. A phaseencoding gradient magnetic field pulse 204 is applied discretely insynchronism with the gradient magnetic field pulse 206. Echo signals 207of each phase encoding are generated in time series within each cycle ofthe inverting read gradient magnetic field 206. The echo signals aresampled within the time range 208 to obtain time series data. The imageis reconstructed from each echo signal so acquired, and image data of64×64 pixels, for example, is acquired. The time range of individual A/Dwindows of sampling 208 is typically about 1 ms, and the adjacent gap ofthe A/D windows is from about 0.5 to about 1 ms. All the echoesnecessary for the image reconstruction are collected by a series ofoperations 209.

The section to be imaged is decided by the gradient magnetic field 202in the slice direction. In this case, the slice selected by each signalacquisition step is a part of the region selected by the inversionlongitudinal magnetization generation step 101, and mutually differentslices (slice 1 to slice 4) are selected serially in the signalacquisition steps 103 to 106. Assuming that the slice thickness is 2.5mm and the slice gap is 0 mm, for example, in each signal acquisitionstep in the inversion longitudinal magnetization generation step 101,the images of the four different slices can be acquired.

In the case of the EPI sequence described above, the echo time TE 211 ofeach signal acquisition step is 10 ms and the length of an echo train207 (for acquiring a plurality of echo signals) is 100 ms, for example.The signal acquisition step can be iterated in the cycle of the sum ofabout 10 ms+100 ms+ wait time, and the images of different inversiontime TI can be acquired. In the example shown in the drawings, theinversion time TI1 of the first slice is 600 ms, TI2=800 ms for thesecond slice, TI3=1,000 ms and TI4=1,200 ms. The iteration time Tr is2,000 ms. This embodiment can acquire four slices in 1,200 ms and fourkinds of TI images.

The second inversion longitudinal magnetization generation step 102 isthe same as the first inversion longitudinal magnetization generationstep 101 with the exception that the slice selecting gradient magneticfield pulse (902 in FIG. 9) is not used. The adiabatic inversion RFpulse is applied under the slice non-selecting state. The subsequentsignal acquisition steps 107 to 110 are exactly the same as the signalacquisition steps 103 to 106 described above, serially select a part ofthe slices (slice 1 to slice 4) of the region selected by the firstinversion longitudinal magnetization generation step 101, and acquirefour image data having different inversion time and the slices by theEPI sequence.

Hatched portions in FIG. 1B show the image data acquired by the signalacquisition steps 103 to 106 and 107 to 110.

In this embodiment, the magnetization excited by the RF irradiation ineach signal acquisition step (imaging step) is contained in eachmutually different slice. In other words, RF irradiation of the slice 1does not excite magnetization of the slice 2. In consequence, even whenthe step 104 is executed in succession to the step 103, saturation oflongitudinal magnetization does not occur during imaging of the slice 2,and the drop of the signal quantity, that is, deterioration of imagequality, does not occur, either. Consequently, each signal acquisitionstep can be executed continuously within a short iteration time. Thoughthe drawing shows the case where four slice images are acquired, thesignal acquisition process of a greater number of steps (e.g. 8 steps)can be executed within the time interval Tr.

As described above, the difference between the data of the same slice isdetermined in the image data acquired by the first and second sequencesand by so doing, the image for each slice can be obtained.

In other words, imaging 103 provides the image of the slice 1 in theinversion time TIl in FIG. 1A, and imaging 104 provides the image of theslice 2 of the inversion time TI2. Imaging 105 provides the image of theslice 3 of the inversion time TI3 and imaging 106, the image of theslice 4 of the inversion time TI4. The signals that reflect thebloodstream (perfusion) from the cerebral artery and are inverted by theIR pulse, are imparted to these images, respectively. Imaging 107 givesthe image of the slice 1 of the inversion time TI1 and imaging 108 givesthe image of the slice 1 of the inversion time TI2. Imaging 109 givesthe image of the slice 3 of the inversion time TI3 and imaging 110 givesthe image of the slice 4 of the inversion time TI4. The signals thatreflect the bloodstream (perfusion) from the cerebral artery inverted bythe IR pulses are imparted to these images, respectively.

The image reflecting the bloodstream (perfusion) is acquired for each ofthese images. The bloodstream (perfusion) information can be selectivelyimaged by conducting the following process. When the image acquired byimaging 107 is subtracted from the image acquired by imaging 103, thedifference of the signal of the object itself is acquired, and only thebloodstream (perfusion) information of the slice 1 at TI1 can beacquired. When the image acquired by imaging 108 is subtracted from theimage acquired by imaging 104, the difference of the signal of theobject itself is acquired, and only the bloodstream (perfusion)information of the slice 2 at TI2 can be acquired. When the imageacquired by imaging 109 is subtracted from the image acquired by imaging105, the difference of the signal of the object itself is acquired, andonly the bloodstream (perfusion) information of the slice 3 at TI3 canbe acquired. When the image acquired by imaging 110 is subtracted fromimaging 106, the difference of the signal of the object itself isacquired, and only the bloodstream (perfusion) information of the slice4 at TI4 can be acquired.

As described above, the embodiment shown in FIG. 1A can acquire aplurality of slice images each a having different inversion time withina short time.

Next, a method of acquiring images each having a mutually differentinversion time for a plurality of slice images will be explained as anembodiment that applies the MRI method shown in FIG. 1A. As shown inFIG. 3A, this method iterates a plurality of sets (four sets, in thedrawing) of sequences 301 to 304 with the sequences shown in FIG. 1Aforming one set. In other words, the first set of sequence 301 comprisesan inversion longitudinal magnetization generation step 101 that selectsa slice, a plurality (four, in this case) of signal acquisition steps103 to 106 subsequent to the step 101, a slice non-selecting inversionlongitudinal magnetization generation step 102, and a plurality ofsignal acquisition steps 107 to 110 subsequent to the step 102. Theseinversion longitudinal magnetization generation steps 101, 102 andsignal acquisition steps 103 to 110 are the same as those of theembodiment explained with reference to FIG. 1A. The difference betweenthe data acquired by the imaging step subsequent to the slice selectinginversion longitudinal magnetization generation step 101 and the dataacquired by the imaging step subsequent to the slice non-selectinginversion longitudinal magnetization generation step 102 is likewisecalculated for each slice.

In this embodiment, however, the sequence of slice selection in thesignal acquisition steps is different between the sets of sequences, andimaging is conducted in such a fashion that the relation ofcorrespondence between the inversion time and the selected slice becomesdifferent. Namely, the slices 1 to 4 are selected serially in the signalacquisition steps 103 to 106 (107 to 110) in the first imaging step 301.In the second imaging step 302, however, the slice 4, next the slice 1,and then the slice 2, and finally the slice 3, are selected in the firstsignal acquisition step. In the third imaging step 303, the slices areselected in the order of the slice 3, the slice 4, the slice 1 and theslice 2. In the fourth imaging step 304, the slices are selected in theorder of the slice 2, the slice 3, the slice 4 and the slice 1.Consequently, four images having mutually different inversion times canbe acquired for the four slices at the end of the fourth imaging step304.

FIG. 3B shows the relationship between the slice position of the imageacquired by each imaging step 301 to 304 and the inversion time.

If the time interval Tr is 2,000 ms, this embodiment can acquire 16images in total for four slices in 8×Tr=16 seconds. In this way, theimages reflecting the moving condition of the bloodstream over a broadrange can be acquired within an extremely short time. Therefore, thoseimages which are very useful for the diagnosis of the disease, thatgenerates the change in the bloodstream in a broad range and has anexpansion, such as the cerebral apoplexy, can be provided.

Each image comprising the combination of TI and the slice positioncontains the slice selecting TI image and the slice non-selecting TIimage. The image that emphasizes the bloodstream (perfusion) informationcan be acquired by conducting the following operation for 4×4×2=32images.

(1) Slice difference is conducted for each of TI=600 ms, the slice 1,slice selection and slice non-selection, and the bloodstream (perfusion)image of TI=600 ms and the slice 1 can be acquired. This also holds trueof other TI values and the slice number.

(2) When the difference is calculated between the images obtained indifferent TI (for example, TI=800 and 1,000) and in the same slice (forexample, the slice 1) among the images obtained in (1), the imagesreflecting the perfusion state in the slice 1 and in 200 ms can beacquired. These images can grasp the perfusion further precisely.

The above explains a plurality of signal acquisition steps subsequent tothe inversion longitudinal magnetization generation step about theembodiment (first embodiment) for acquiring the signals by making theselecting slices different. However, the MRI method according to thepresent invention may be executed by selecting the same slice for aplurality of signal acquisition steps. Next, another embodiment (secondembodiment) of the invention for executing a plurality of signalacquisition steps for the same slice will be explained.

The MRI method of the embodiment shown in FIG. 5 comprises the inversionlongitudinal magnetization generation step 101 that selects the slice, aplurality (four, in this embodiment) of signal acquisition steps 503 to506 subsequent to the step 101, a slice non-selecting inversionlongitudinal magnetization generation step 102 and a plurality ofsubsequent signal acquisition steps 507 to 510. Each of the signalacquisition steps executes single-shot EPI 503 to 510 such as the oneshown in FIG. 2 in the same way as in the embodiment shown in FIG. 1A.The difference of this embodiment from the embodiment shown in FIG. 1Ais that each signal acquisition step images the same slice. Therefore,the slice selected in the inversion longitudinal magnetizationgeneration step 101 is the same as the slice that is selected in thesubsequent signal acquisition steps, or as the slice that contains theformer.

In this embodiment, the same slice is excited consecutively. Therefore,the RF pulse applied in each EPI sequence is preferably smaller than 90degrees. A de-phase pulse 212 (represented by dotted line in FIG. 2) isapplied, whenever necessary, to the last part of each EPI sequence inorder to remove the influences of the gradient magnetic field pulsesapplied during the process steps.

The difference is calculated between the image data acquired from theecho signals given by the EPI sequences 503 to 506 and the image dataacquired from the echo signals given by the EPI sequences 507 to 510 foreach different inversion time TI. In this way, a plurality of imageseach having a different inversion time can be acquired consecutively forthe same slice. The images each having a different inversion timereflect the moving region of the bloodstream as has already beendescribed. Therefore, this embodiment can image the mode of the movementof the bloodstream inside the same slice within a short time. To acquirethe images for the multiple slices, the sequence shown in FIG. 5 isiterated while the slices to be selected are being changed.

In comparison with the embodiment shown in FIG. 1A, this embodimentinvolves the case where the signal quantity drops because relaxation oflongitudinal magnetization is not entirely complete. None the less, thisembodiment can acquire the images each having a different inversion timefor the same slice, can shorten the imaging time and can improve timeresolution in comparison with the prior art methods.

Each signal acquisition step in this embodiment executes single-shortEPI and acquires the image having mutually different inversion time.However, a single image may be acquired by a plurality of signalacquisition steps by executing multi-shot EPI in these signalacquisition steps. Such an embodiment is shown in FIG. 6. In thisembodiment, the individual signal acquisition steps 603 to 610 execute asplit type EPI.

In the operation of EPI shown in FIG. 2, the split type EPI acquiresonly a part of the phase encode data, iterates the operation 209 whilethe pulse 203 that provides the offset of the phase encode (representedby the dotted line in the drawing) is changed, and acquires theremaining echo signals 207. Referring to FIG. 6, the same slice isselected in the continuous signal acquisition steps 603 to 606. Whilethe phase encode pulses 203 are serially changed, the split type EPI isexecuted to acquire the echo signals by the different phase encodes.Finally, the echo signals necessary for reconstructing one image for theslice 1 are acquired by the four signal acquisition steps. Incidentally,the RF pulse of each EPI is made smaller than 90 degrees, and a crusherpulse or a de-phase pulse 212 is added after the operation 209, whenevernecessary.

In FIG. 6, the imaging step 603 acquires ¼ of the echoes necessary forthe image acquisition from the echoes acquired by the IR sequence of theinversion time TI1. This will be called the “phase encode 1”. Theimaging step 604 acquires {fraction (2/4)} of the echoes necessary forthe image acquisition from the echoes acquired by the IR sequence of theinversion time TIl. This will be called the “phase encode 2”. Theimaging step 605 acquires ¾ of the echoes necessary for the imageacquisition from the echoes acquired by the IR sequence of the inversiontime TI1. This will be called the “phase encode 3”. The imaging step 606acquires {fraction (4/4)} of the echoes necessary for the imageacquisition from the echoes acquired by the IR sequence of the inversiontime TI1. This will be called the “phase encode 4”. These phase encodes1 to 4 are transformed by a known signal processing (two-dimensionalFourier transform) and give the image of the mean TI. Here, the mean TIis given approximately by (TI1+TI2+TI3+TI4)/4.

The signal acquisition steps 607 to 610 of the latter half also executethe split type EPI. The difference is calculated between the image dataacquired at this time and the image data acquired by the signalacquisition steps 603 to 606 of the former half.

The number of echoes acquired in the operation 209 of the EPI sequenceshown in FIG. 2 becomes smaller in the split type (multi-shot type) EPIthan in the single-shot EPI. Therefore, the time of one signalacquisition step becomes 209 shorter. As a result, the drop of the echosignals in the latter half of the operation 209 becomes smaller, and theimages having higher quality can be acquired. Since the quantity of thephase encodes becomes greater (twice) in comparison with the embodimentshown in FIG. 5, the images having higher spatial resolution can beacquired. In the embodiment (multi-shot EPI) shown in FIG. 6, however,the inversion time TI has an effective TI which is the mean of theinversion time TI1 to TI4 of each signal acquisition step.

Next, the embodiment wherein the present invention is applied to athree-dimensional imaging sequence will be explained as still anotherembodiment (third embodiment) of the present invention. As shown in FIG.7, this embodiment comprises a slice selecting inversion longitudinalmagnetization generation step 101, a slice non-selecting inversionlongitudinal magnetization generation step 102 and a plurality (four, inthis case) of subsequent signal acquisition steps 703 to 706, and 707 to710. The signal acquisition steps 703 to 710 are three-dimensional EPIfor imaging the same slab (thick slice). In the EPI sequence shown inFIG. 2, the three-dimensional EPI selects the slabs by the slicegradient magnetic field 202 applied simultaneously with the RF pulse201, adds the slice encode pulse 213 represented by the dotted line, anditerates the operation that is executed within the time 209 while theslice encode pulse 213 is changed for each RF pulse 201. A crusher pulse212 may be added, whenever necessary.

In the embodiment shown in FIG. 7, each of the signal acquisition stepsapplies a different slice encode gradient magnetic field (slice encode 1to slice encode 4) and acquires the signal. In this case, too, the RFpulse of each EPI is smaller than 90 degrees, and a de-phase pulse ispreferably added to the last part of each EPI, whenever necessary. Thedifference is calculated between the three-dimensional image data fromthe signals acquired by the former half signal acquisition steps 703 to706 and the three-dimensional image data from the signals acquired bythe latter half signal acquisition steps 707 to 710. Thethree-dimensional image is reconstructed in this way. The inversion timeTI of this image is the mean of the inversion time TI1 to TI4 of eachsignal acquisition means.

This embodiment iterates the excitation of the same slab in the same wayas in the embodiment (iteration of the single-shot EPI) shown in FIG. 5.Therefore, the signal quantity drops in some cases because therelaxation of longitudinal magnetization is not complete. However, thisembodiment has the merit that spatial resolution in the slice directionbecomes higher than the multi-slice single-shot EPI that iterates thesequence of FIG. 5 by changing the selecting slice. It has also themerit that image quality can be improved.

The MRI method according to the present invention has thus beenexplained about the case 1) where different slices are selected by aplurality of signal acquisition steps (FIGS. 1A and 3A), and the case 2)where the same slice or slab is selected (FIGS. 5 to 7). In these cases,the single-shot or split type EPI is executed by the signal acquisitionsteps of each embodiment. However, other sequences of EPI may beemployed as the signal acquisition sequence. For example, a sequencereferred to as a “burst sequence” using a plurality of RF pulses as theexcitation pulse, and a “hybrid burst sequence” comprising thecombination of this burst sequence with the inversion of the gradientmagnetic field, can be employed, too.

FIG. 8 shows an example of the hybridburst sequence. In this sequence, aplurality (five, in FIG. 8) of RF pulses 1001 are irradiatedcontinuously while a readout pulse 1002 is being applied. Next, a 180°RF pulse 1003 is irradiated while a slice gradient magnetic field 1004is being applied. When a readout pulse 1005 having the same shape as thereadout pulse 1002 is irradiated, the same number of echoes 1007 as thenumber of irradiated RF pulses 1001 develop. A phase encode pulse 1006is applied in synchronism with collection of the echoes. When inversionreadout pulses 1008 are applied subsequently, the same number of echoes1009 further develop. Data sampling 1010 is executed in match with theechoes.

The burst sequence generates a plurality of echoes by the continuous RFpulses unlike the EPI. Therefore, it can collect the signals within ashort time. Particularly because the hybrid burst sequence shown in thedrawing partly uses the inversion gradient magnetic field, too, it canfurther shorten the imaging time 1011 in comparison with the EPI. Atypical imaging time is 20 to 40 ms.

In the example shown in the drawing, the inversion gradient magneticfields (1005 and 1008) constitute one set. In order to increase thenumber of phase encodes, however, the inversion magnetic fields may beiterated continuously such as eight times. In this way, the echo signalsnecessary for one image can be acquired by executing once the sequencein the same way as the single-shot EPI. Such a hybrid burst sequence canbe employed as the sequence of the signal acquisition steps of theembodiments shown in FIGS. 1A, 3A, 5 and 7. Moreover, because theimaging time is as fast as 20 ms, a greater number of TI images and agreater number of multi-slice images can be acquired.

For, in the case of the perfusion measurement, the TI range of fromabout 800 ms to about 1,200 ms is believed clinically significant. Ifthe time necessary for the imaging process is short as in burst imaging,a greater number of images can be acquired within a given time. Thisincrement may be utilized for increasing the number of setting of TI,for example. In this case, the time change of perfusion can be imaged indetail. It may also be utilized for increasing the number of slices. Inthis case, the spatial distribution of the perfusion in the slicedirection can be imaged in detail.

Incidentally, when the hybrid burst sequence is applied tothree-dimensional imaging shown in FIG. 7, the slice gradient magneticfield 1004 is utilized as the slab selecting gradient magnetic field,and then the slice encode pulses are added. The signal acquisition stepsare iterated while the slice encode pulses are being changed.

The hybrid burst sequence shown in FIG. 8 may be converted to themulti-shot sequence by combining a plurality of times the series ofoperations executed in the time 1011 in order to increase the number ofphase encodes. The number of iteration of the operations is four, forexample, and each operation acquires ten echoes (1007 and 1009). Theoperations of four times acquire 40 echoes in total, or all the echosignals necessary for reconstructing one image. A known technology isemployed for converting the signals to the image. For example, data ofrelatively low phase encodes are collected by 40 echoes described above,and data of high phase encodes are substituted by zero. The samplingnumber of A/D conversion of each echo is 128, for example. The image isgenerated by two-dimensional Fourier transform of the 60×128 data matrixso prepared. Both ends in the data sampling direction are cut off, andthe 64×64 images can be acquired.

The sequence converted to the multi-shot sequence as described aboveacquires the echoes to which the different phase encode is added foreach shot. Therefore, additional pulses other than those shown in FIG. 8are necessary for the phase encode gradient magnetic field Ge. Thiscorresponds to the case where the application area (intensity×time) ofthe pulse 203 applied for achieving the multi-shot sequence in the echoplanar sequence (FIG. 2) is changed for each shot. In other words, apulse (1012), the application area (intensity×time) of which is changedfor each shot, is added before the application of the phase encode pulse1006 in FIG. 8.

Such a multi-shot hybrid sequence can be employed as the sequence of thesignal acquisition steps of the embodiment shown in FIG. 6. Theindividual signal acquisition steps 603 to 606 correspond to a series ofthe operations executed within the time 1011 of the hybrid burstsequence, respectively.

When the burst sequence, particularly the hybrid burst sequence, isemployed as the sequence of the signal acquisition steps of the presentinvention, the imaging time is as fast as 20 ms. Therefore, a greaternumber of TI images and a greater number of multi-slice images can beacquired than when the EPI sequence is employed.

The present invention can acquire the IR images of different slices byapplying once the IR pulse. The sizes of the region for generatinginversion longitudinal magnetization may be mutually different in theapplication of the slice selecting IR pulses and in the slicenon-selecting IR pulses. These regions may overlap with one another atleast partially, or may be entirely different regions.

The application of the slice selecting IR pulses is made before theapplication of the slice non-selecting IR pulses in each of theforegoing embodiments, but this order may be reversed.

The present invention is not particularly limited to the embodimentsgiven above but includes various changes and modifications within thescope of the appended claims.

What is claimed is:
 1. A nuclear magnetic resonance imaging methodincluding a first inversion longitudinal magnetization generation stepfor generating inversion longitudinal magnetization by applying radiofrequency pulses to an object, and a first imaging step for executingcontinuously a plurality of first signal acquisition steps in successionto said first inversion longitudinal magnetization generation step:wherein each of a plurality of said first signal acquisition stepcomprises the steps of: irradiating at least one radio frequency pulseto said object and generating transverse magnetization in a selectedslice region; generating a plurality of echo signals while phaseencoding gradient magnetic fields are being applied to said object towhich said transverse magnetization is applied; and detecting aplurality of said echo signals in a time series while a read gradientmagnetic field is applied to said object, a polarity of said readgradient magnetic field being alternately changed; and wherein aplurality of said first signal acquisition steps select mutuallydifferent slice positions.
 2. A nuclear magnetic resonance imagingmethod according to claim 1, wherein said first inversion longitudinalmagnetization generation step generates said inversion longitudinalmagnetization in a region containing all the slice positions selected bya plurality of said first signal acquisition steps, respectively.
 3. Anuclear magnetic resonance imaging method including a first inversionlongitudinal magnetization generation step for generating inversionlongitudinal magnetization by applying radio frequency pulses to anobject, and a first imaging step for executing continuously a pluralityof first signal acquisition steps in succession to said first inversionlongitudinal magnetization generation step: wherein each of a pluralityof said first signal acquisition step comprises the steps of:irradiating at least one radio frequency pulse to said object andgenerating transverse magnetization in a selected slice region;generating a plurality of echo signals while phase encoding gradientmagnetic fields are being applied to said object to which saidtransverse magnetization is applied; and detecting a plurality of saidecho signals in a time series; and wherein a plurality of said firstsignal acquisition steps select mutually different slice positions; asecond inversion longitudinal magnetization generation step forirradiating radio frequency pulses to said object and generatinginversion longitudinal magnetization; and a second imaging step forexecuting continuously a plurality of second signal acquisition steps insuccession to said second inversion longitudinal magnetizationgeneration step; wherein each of a plurality of said second signalacquisition steps includes: a step for irradiating at least one radiofrequency pulse to said object and generating transverse magnetizationin a selected slice region; a step for generating a plurality of echosignals while imparting phase encode gradient magnetic fields to saidobject having said transverse magnetization applied thereto; and a stepfor detecting a plurality of said echo signals in a time series; andwherein a plurality of said second signal acquisition steps selectmutually different slice positions selected by said first imaging step.4. A nuclear magnetic resonance imaging method according to claim 3,wherein said first and second inversion longitudinal magnetizationgeneration steps include a step for generating inversion longitudinalmagnetization in regions having mutually different sizes.
 5. A nuclearmagnetic resonance imaging method according to claim 4, furthercomprising: a step for determining difference data for the same slicebetween said echo signals acquired by said first imaging step and saidecho signals acquired by said second imaging step; and a step forreconstructing an image on the basis of said difference data.
 6. Anuclear magnetic resonance imaging method according to claim 4, furthercomprising: a step of iterating a set of process steps of said firstinversion longitudinal magnetization step, said first imaging step, saidsecond inversion longitudinal magnetization generation step and saidsecond imaging step for a plurality of sets; wherein an order of saidslices selected by said first imaging step and by said second imagingstep is serially changed in the iteration of a plurality of said sets.7. A nuclear magnetic resonance imaging method according to claim 6,further comprising: a step for determining difference data for the sameslice between said echo signals acquired by said first imaging step andsaid echo signals acquired by said second imaging step; and a step forreconstructing said image on the basis of said difference data.
 8. Anuclear magnetic resonance imaging method including a first inversionlongitudinal magnetization generation step for generating inversionlongitudinal magnetization by irradiating radio frequency pulses to anobject, a first imaging step for executing continuously a plurality offirst signal acquisition steps after said first inversion longitudinalmagnetization generation step, a second inversion longitudinalmagnetization generation step for generating inversion longitudinalmagnetization by applying radio frequency pulses to said object, and asecond imaging step for executing continuously a plurality of secondsignal acquisition steps after said second inversion longitudinalmagnetization generation step; wherein each of a plurality of said firstand second signal acquisition steps includes: a step for irradiating atleast one radio frequency pulse to said object and generating transversemagnetization in a selected slice region; a step for generating aplurality of echo signals while applying phase encode gradient magneticfields to said object having said transverse magnetization appliedthereto; and a step for detecting a plurality of said echo signals in atime series; wherein said first and second inversion longitudinalmagnetization generation steps generate said inversion longitudinalmagnetization in regions having mutually different sizes; wherein aplurality of said first and second signal acquisition steps selectingone and the same slice position; and wherein an inversion time TI1, TI2,. . . , TIn (where n is an integer) from the end point of said firstinversion longitudinal magnetization step to each of a plurality of saidfirst signal acquisition steps is the same as the inversion time fromthe end point of said second inversion longitudinal magnetizationgeneration step to the irradiation of said radio frequency pulse in eachof a plurality of said second signal acquisition steps.
 9. A nuclearresonance imaging method according to claim 8, further comprising: astep for determining difference data for the same inversion time betweensaid echo signals acquired by said first imaging step and said echosignals acquired by said second imaging step; and a step forreconstructing said image on the basis of said difference data.
 10. Anuclear magnetic imaging method including a first inversion longitudinalmagnetization generation step for generating inversion longitudinalmagnetization by irradiating radio frequency pulses to an object, afirst imaging step for executing continuously a plurality of firstsignal acquisition steps after said first inversion longitudinalmagnetization generation step, a second inversion longitudinalmagnetization generation step for generating inversion longitudinalmagnetization by irradiating radio frequency pulses to said object aftersaid first imaging step, and a second imaging step for executingcontinuously a plurality of second signal acquisition steps after saidsecond inversion longitudinal magnetization generation step; whereineach of a plurality of said first and second signal acquisition stepsincludes: a step for irradiating at least one radio frequency pulse tosaid object and generating transverse magnetization in a selected sliceregion; a step for generating a plurality of echo signals while phaseencode gradient magnetic fields are being applied to said object havingsaid transverse magnetization applied thereto; and a step for detectinga plurality of said echo signals in a time series; and wherein saidfirst and second inversion longitudinal magnetization generation stepsgenerate said inversion longitudinal magnetization in regions havingmutually different sizes, and a plurality of said first and secondsignal acquisition steps include a step for selecting one and the sameslice position and applying mutually different phase encode pulses. 11.A nuclear magnetic resonance imaging method according to claim 10,further comprising: a step for transforming said echo signals acquiredby said first imaging step by two-dimensional Fourier transform andacquiring first image data; a step for transforming said echo signalsacquired by said second imaging step by two-dimensional Fouriertransform and acquiring second image data; and a step for reconstructingone image on the basis of difference data between said first and secondimage data.
 12. A nuclear magnetic resonance imaging method including afirst inversion longitudinal magnetization generation step forgenerating inversion longitudinal magnetization by irradiating radiofrequency pulses to an object containing magnetization to be detected, afirst imaging step for executing continuously a plurality of firstsignal acquisition steps after said first inversion longitudinalmagnetization generation step, a second inversion longitudinalmagnetization generation step for generating inversion longitudinalmagnetization by irradiating radio frequency pulses to said object aftersaid first imaging step, and a second imaging step for executingcontinuously a plurality of second signal acquisition steps after saidsecond inversion magnetization generation step: wherein each of aplurality of said first and second signal acquisition steps includes: astep for irradiating at least one radio frequency pulse to said objectand generating transverse magnetization in a selected slice region; astep for applying mutually different slice encode gradient magneticfields to said object having said transverse magnetization appliedthereto; a step for applying different phase encode gradient magneticfields to said object having said transverse magnetization appliedthereto, and generating a plurality of echo signals; and a step fordetecting a plurality of said echo signals in a time series; and whereineach of said first and second inversion longitudinal magnetizationgeneration steps generates said inversion longitudinal magnetization inregions having mutually different sizes.
 13. A nuclear magneticresonance imaging method according to claim 12, further including: astep for transforming said echo signals acquired by said first imagingstep by three-dimensional Fourier transform, and acquiring firstthree-dimensional image data; a step for transforming said echo signalsacquired by said second imaging step by three-dimensional Fouriertransform, and acquiring second three-dimensional image data; and a stepfor reconstructing one three-dimensional image on the basis ofdifference data between said first and second image data.
 14. A nuclearmagnetic resonance imaging method according to any of claims 3, 8, 10and 12, wherein a plurality of said first and second signal acquisitionsteps irradiate a plurality of radio frequency pulses while gradientmagnetic fields in a readout direction are being applied, then irradiate18011 radio frequency pulses while slice selecting gradient magneticfields are being applied, serially generate the same number of sets ofsaid echo signals corresponding to a plurality of said radio frequencypulses while said gradient magnetic fields in the readout direction arebeing inverted, apply in this instance mutually different phase encodegradient magnetic field pulses whenever said gradient magnetic fields inthe readout direction invert, and detect said sets of a plurality ofsaid echo signals in a time series.
 15. A nuclear magnetic resonanceimaging apparatus including a static magnetic field generation unit, agradient magnetic field generation unit, a radio frequency pulsegeneration unit, a signal detection unit, a signal processing unit, adisplay unit and a control unit for controlling the overall operationsof said nuclear magnetic resonance imaging apparatus: wherein saidcontrol unit executes; first control for irradiating radio frequencypulses to an object and generating first inversion longitudinalmagnetization; second control for conducting control in such a manner asto execute a plurality of first signal acquisition sequences insuccession to said first inversion longitudinal magnetizationgeneration; and wherein said second control is executed so that at leastone radio frequency pulse is irradiated to said object in each of aplurality of said first signal acquisition sequences, transversemagnetization is generated in a selected slice region, a plurality ofecho signals are generated while phase encode gradient magnetic fieldsare being applied to said object having said transverse magnetizationapplied thereto, said signal detection unit is caused to detect aplurality of said echo signals in time series while a read gradientmagnetic field is applied to said object, a polarity of said readgradient magnetic field being alternately changed, and mutuallydifferent slice positions are selected in each of a plurality of saidfirst signal acquisition sequences.
 16. A nuclear magnetic resonanceimaging apparatus according to claim 15, wherein said first control isexecuted so that said first inversion longitudinal magnetization isgenerated in a region containing all the slice positions selected by aplurality of first signal acquisition sequences.
 17. A nuclear magneticresonance imaging apparatus including a static magnetic field generationunit, a gradient magnetic field generation unit, a radio frequency pulsegeneration unit, a signal detection unit, a signal processing unit, adisplay unit and a control unit for controlling the overall operationsof said nuclear magnetic resonance imaging apparatus: wherein saidcontrol unit executes; first control for irradiating radio frequencypulses to an object and generating first inversion longitudinalmagnetization; second control for conducting control in such a manner asto execute a plurality of first signal acquisition sequences insuccession to said first inversion longitudinal magnetizationgeneration; and wherein said second control is executed so that at leastone radio frequency pulse is irradiated to said object in each of aplurality of said first signal acquisition sequences, transversemagnetization is generated in a selected slice region, a plurality ofecho signals are generated while phase encode gradient magnetic fieldsare being applied to said object having said transverse magnetizationapplied thereto, said signal detection unit is caused to detect aplurality of said echo signals in time series, and mutually differentslice positions are selected in each of a plurality of said first signalacquisition sequences; wherein said control unit further executes: athird control for irradiating radio frequency pulses to said object andgenerating second inversion longitudinal magnetization; and a fourthcontrol for executing continuously a plurality of second signalacquisition sequences in succession to said second inversionlongitudinal magnetization generation; and wherein said fourth controlis executed so that at least one radio frequency pulse is irradiated tosaid object in each of a plurality of said second signal acquisitionsequences, transverse magnetization is generated in a selected sliceregion, a plurality of echo signals are generated while phase encodegradient magnetic fields are applied to said object having saidtransverse magnetization applied thereto, a plurality of said echosignals are detected in a time series by said signal detection unit, anda plurality of said second signal acquisition sequences select mutuallydifferent slice positions selected by a plurality of said first signalacquisition sequences.
 18. A nuclear magnetic resonance imagingapparatus according to claim 17, wherein said first and third controlsare executed so that said inversion longitudinal magnetization isgenerated in regions having mutually different sizes.
 19. A nuclearmagnetic resonance imaging apparatus according to claim 18, wherein saidsignal processing unit includes: a first processing unit for determiningdifference data for the same slice between said echo signals acquired bya plurality of said first signal acquisition sequence and said echosignals acquired by a plurality of said second signal sequence; and asecond processing unit for reconstructing an image on the basis of saiddifference data.
 20. A nuclear magnetic resonance imaging apparatusaccording to claim 18, wherein said control unit executes control sothat said first inversion longitudinal magnetization generation, aplurality of said first signal acquisition sequences, said secondinversion longitudinal magnetization generation and a plurality of saidsignal acquisition sequences are iterated as one set for a plurality ofsets, and an order of slices selected in a plurality of said firstsignal acquisition sequences and in a plurality of said second signalacquisition sequences is serially changed during the iteration of aplurality of said sets.
 21. A nuclear magnetic resonance imagingapparatus according to claim 20, wherein said signal processing unitincludes: a first processing unit for determining difference data forthe same slice between said echo signals acquired by a plurality of saidfirst signal acquisition sequences and said echo signals acquired by aplurality of said signal acquisition sequences; and a second processingunit for reconstructing an image on the basis of said difference data.22. A nuclear magnetic resonance imaging apparatus including a staticmagnetic field generation unit, a gradient magnetic field generationunit, a radio frequency pulse generation unit, a signal detection unit,a signal processing unit, a display unit and a control unit forcontrolling the overall operations of said nuclear resonance imagingapparatus: wherein said control unit executes: first control forirradiating radio frequency pulses to an object and generating firstinversion longitudinal magnetization; second control for executingcontinuously a plurality of first signal acquisition sequences insuccession to said first inversion longitudinal magnetizationgeneration; third control for irradiating radio frequency pulses to saidobject after a plurality of said first signal acquisition sequences andgenerating second inversion longitudinal magnetization; and fourthcontrol for executing continuously a plurality of second signalacquisition sequences in succession to said second inversionlongitudinal magnetization generation; said first and third controlsbeing made so that said inversion longitudinal magnetization isgenerated in regions having mutually different sizes; said second andfourth controls being made so that at least one radio frequency pulse isapplied to said object in each of a plurality of said first and secondsignal acquisition sequences to generate transverse magnetization in aselected slice region, a plurality of echo signals are generated whilephase encode gradient magnetic fields are being applied to said objecthaving said transverse magnetization applied thereto, said signaldetection unit detects a plurality of said encode signals in a timeseries, said first and second signal acquisition sequences select oneand the same slice position, and an inversion time TI1, TI2, . . . , TIn(where n is an integer) from the end point of said first inversionlongitudinal magnetization generation to the irradiation of said radiofrequency pulse in each of a plurality of said first signal acquisitionsequences is the same as an inversion time from the end point of saidsecond inversion longitudinal magnetization generation to theirradiation of said radio frequency pulse in each of a plurality of saidsecond signal acquisition sequence.
 23. A nuclear magnetic resonanceimaging apparatus according to claim 22, wherein said signal processingunit includes: a first processing unit for determining difference datafor the same inversion time between said echo signals acquired by aplurality of said first signal acquisition sequences and said echosignals acquired by a plurality of said second signal acquisitionsequences; and a second processing unit for reconstructing an image onthe basis of said difference data.
 24. A nuclear magnetic resonanceimaging apparatus including a static magnetic field generation unit, agradient magnetic field generation unit, a radio frequency pulsegeneration unit, a signal detection unit, a signal processing unit, adisplay unit and a control unit for controlling the overall operationsof said nuclear magnetic resonance apparatus; wherein said control unitexecutes: first control for irradiating radio frequency pulses to anobject and generating first inversion longitudinal magnetization; secondcontrol for executing continuously a plurality of first signalacquisition sequences in succession to said first inversion longitudinalmagnetization generation; third control for irradiating radio frequencypulses to said object after a plurality of said first signal acquisitionsequences and generating second inversion longitudinal magnetization;fourth control for executing continuously a plurality of second signalacquisition sequences in succession to said second inversionlongitudinal magnetization; wherein said first and third controls aremade in such a fashion that said inversion longitudinal magnetization isgenerated in regions having mutually different sizes; and wherein saidsecond and fourth controls are made in such a fashion that at least oneradio frequency pulse is irradiated to said object in each of aplurality of said first and second signal acquisition sequences togenerate transverse magnetization in a selected slice region, aplurality of echo signals are generated while phase encode gradientmagnetic fields are being applied to said object having said transversemagnetization applied thereto, said signal detection unit detects aplurality of said echo signals in a time series, a plurality of saidfirst and second signal acquisition sequences acquire one and the sameslice position, and mutually different phase encode gradient magneticfields are applied.
 25. A nuclear magnetic resonance imaging apparatusaccording to claim 24, wherein said signal processing unit includes: afirst processing unit for transforming said echo signals acquired bysaid first signal acquisition sequences by two-dimensional Fouriertransform and acquiring first image data; a second processing unit fortransforming said echo signals acquired by said second signalacquisition sequences by two-dimensional Fourier transform and acquiringsecond image data; and a third processing unit for reconstructing oneimage on the basis of difference data on the basis of first and secondimage data.
 26. A nuclear magnetic resonance imaging apparatus includinga static magnetic field generation unit, a gradient magnetic fieldgeneration unit, a radio frequency pulse generation unit, a signaldetection unit, a signal processing unit, a display unit and a controlunit for controlling the overall operations of said nuclear magneticresonance imaging apparatus, wherein said control unit executes: firstcontrol for irradiating radio frequency pulses to an object andgenerating first inversion longitudinal magnetization; second controlfor executing continuously a plurality of first signal acquisitionsequences in succession to said first inversion longitudinalmagnetization generation; third control for irradiating radio frequencypulses to said object after a plurality of said first signal acquisitionsequence and generating second inversion longitudinal magnetization; andfourth control for executing continuously a plurality of second signalacquisition sequences in succession to said second inversionlongitudinal magnetization generation; wherein said second and fourthcontrols are made in such a fashion that at least one radio frequencypulse is irradiated to said object in each of a plurality of said firstand second signal acquisition sequences to generate transversemagnetization in a selected slice region, mutually different sliceencode gradient magnetic fields are applied to said object having saidtransverse magnetization applied thereto, and said signal detection unitdetects said echo signals in a time series; and wherein said first andthird control are made in such a fashion that said inversionlongitudinal magnetization is generated in regions having mutuallydifferent sizes.
 27. A nuclear magnetic resonance imaging apparatusaccording to claim 26, wherein said signal processing unit includes: afirst processing unit for transforming echo signals acquired by saidfirst signal acquisition sequences by three-dimensional Fouriertransform and acquiring first three-dimensional image data; a secondprocessing unit for transforming echo signals acquired by said secondsignal acquisition sequences by three-dimensional Fourier transform andacquiring second three-dimensional image data; and a third processingunit for reconstructing one three-dimensional image on the basis ofdifference data between said first and second three-dimensional imagedata.
 28. A nuclear magnetic resonance imaging apparatus according toany of claims 17, 22, 24 and 26, wherein said control unit executescontrol in such a fashion that a plurality of radio frequency pulses areapplied while gradient magnetic fields in a readout direction are beingapplied, in each of a plurality of said first and second signalacquisition sequences, 180° radio frequency pulses are then irradiatedwhile slice selecting gradient magnetic fields are being applied, setsof echo signal in the same number as the number of a plurality of saidradio frequency pulses are serially generated while said gradientmagnetic fields in the readout direction are being inverted, a differentphase encode gradient magnetic field pulse is applied in this instancewhenever said gradient magnetic fields in the readout direction invert,and a plurality of sets of said echo signals are detected in a timeseries.
 29. A nuclear magnetic resonance imaging method including afirst inversion longitudinal magnetization generation step forgenerating inversion longitudinal magnetization by applying radiofrequency pulses to an object, and a first imaging step for executingcontinuously a plurality of first signal acquisition steps in successionto said first inversion longitudinal magnetization generation step:wherein each of a plurality of said first signal acquisition stepscomprises the steps of: irradiating a plurality of radio frequencypulses while a gradient magnetic field in a readout direction is beingapplied; irradiating a 180° radio frequency pulse while a sliceselecting gradient magnetic field is being applied; serially generatinga plurality of echo signals corresponding to a plurality of said radiofrequency pulses while a gradient magnetic field in a readout directionis alternately inverted and applied, and different phase encode gradientmagnetic field pulses are applied at each inversion of said gradientmagnetic fields in the readout direction; and detecting said echosignals corresponding to said radio frequency pulses in time series foreach inversion of said gradient magnetic field in the readout direction.30. A nuclear magnetic resonance imaging apparatus including a staticmagnetic field generation unit, a gradient magnetic field generationunit, a radio frequency pulse generation unit, a signal detection unit,a signal processing unit and a control unit for controlling overalloperations of said nuclear magnetic resonance imaging apparatus: whereinsaid control unit executes: first control for irradiating radiofrequency pulses to an object and generating first inversionlongitudinal magnetization; and second control for conducting control insuch a manner as to execute continuously a plurality of first signalacquisition sequences in succession to said first inversion longitudinalmagnetization generation; wherein said second control is executed sothat each of said plurality of said first signal acquisition sequencesincludes irradiating a plurality of radio frequency pulses while agradient magnetic field in a readout direction is applied, irradiating a180° radio frequency pulse while a slice selecting gradient magneticfield is applied, serially generating a plurality of echo signalscorresponding to a plurality of said radio frequency pulses while agradient magnetic field in a readout direction is alternately invertedand applied with different phase encode gradient magnetic fields beingapplied at each inversion of said gradient magnetic fields in thereadout direction, and said signal detection unit detects said echosignals corresponding to said radio frequency pulses in time series foreach inversion of said gradient magnetic field in the readout direction.